Enhanced Data rates for GSM Evolution (EDGE) is standardized as part of Third Generation Partnership Project (3GPP)/Global System for Mobile Telecommunications (GSM)/EDGE Radio Access Network (GERAN). GERAN/EDGE Evolution could be seen as a new packet of features as part of release 7 of the GERAN specifications. One part of that package is called EGPRS2, which introduces higher order modulations and higher symbols rates for both uplink and downlink. Another part of that package is called Reduced Latency and reduces the Time Transmit Interval (TTI) from 20 ms to 10 ms. The feature, often called Reduced TTI (RTTI), currently puts requirements on the downlink transmission in order to still support legacy pre-release 7 terminals. One such requirement forces two consecutive downlink blocks to be transmitted using the same modulation technique. Even though the blocks can be addressed to two different terminals, they both have to be transmitted using either of Gaussian Minimum Shift Keying (GMSK), 8-Phase-shift keying (8PSK), Quadrature Phase-Shift Keying (QPSK), 16-Quadrature Amplitude Modulation (16QAM) or 32-Quadrature Amplitude Modulation (32QAM) in order to be able to schedule a legacy terminal for transmission in the coming period.
The reason for this requirement is the downlink parameter called Uplink State Flag (USF) used to grant a particular terminal access to one, or more, uplink radio block(s). No matter what is sent on the downlink, the legacy as well as the RTTI terminal must be able to decode the USF if intended for the terminal.
The USF could be sent in both RTTI mode, or the legacy way, in Basic Transmission Time Interval (BTTI) mode i.e. mapped over four consecutive Time Division Multiple Access (TDMA) frames, thus e.g. 20 ms. The requirement on using the same modulation, as mentioned above, only applies to the BTTI USF mode when the USF is set to a used value. For RTTI USF mode any combinations of modulations on the two blocks are allowed. This is illustrated in FIG. 1. The left part of FIG. 1 shows the case when legacy radio blocks are sent in the downlink and the right the case when RTTI radio blocks are sent. As can be seen in the right part of FIG. 1, the USF is sent across two different downlink radio blocks. Using the existing technical solution, the USF bits must be sent using one and the same modulation in both radio blocks and hence, the two radio blocks must be sent using the same modulation.
The problem with the existing solution is that the ability to support legacy terminals puts requirements on the downlink scheduler. The downlink scheduler needs to use the same modulation in two consecutive blocks. It frequently results in non-optimal choice of modulation for any or both of the two downlink blocks. This problem is sometimes called modulation segregation.